The television broadcast industry is especially focusing at this time on the development of digital television equipment, and, as part of that program, it seeks to develop new products for TV stations special needs. In this regard, the FCC has issued a large number of new licenses to stations in the United States for digital television (DTV) broadcast, such licenses being in the range from 50 KW to 1,000 KW of effective radiated power (ERP). However, there are within the noted range a group of 680 stations or so which operate at ERP between 50 and 100 KW.
It has been suggested that the particular group of 680 stations or so could radiate power at as much as 1,000 KW as long as they directed that power so that it did not produce a field at their assigned Grade B contour that was greater than the equivalent of their current allotted DTV power. In order for these stations not to produce a field outside their Grade B contour that would be greater than their allotted effective radiated power (ERP), high electrical beam tilts would be necessary. For example, if a station has an antenna which is for 1,500 feet above the average terrain, the Grade B contour will lie at a depression angle of approximately 0.6 degrees below the horizontal. If the allotted ERP at this point is 50 KW, it can be seen by reference to FIG. 7 herein that the power levels inside the assigned Grade B contour, referred to above, can effectively be increased by 7 DB and thus become competitive with more powerful stations in the same area of coverage.
It might be thought that due to the characteristics of a traveling wave type antenna, it would be possible to achieve the desired high electrical beam tilts by simply designing large phase spreads along the illumination aperture of the antenna. But such an approach requires a sophisticated analysis to meet fully the needs of the users whose power outputs are limited.
Accordingly, it is a primary object of the present invention to provide a traveling wave antenna that will establish the required high beam tilt necessary for the given context, but will satisfy the requirements of keeping the fields produced within the required Grade B contour as described.
Another object is to produce an elevation pattern with the required high beam tilt and with substantially no null variations, that is to say to have the null variations, as exemplified by FIG. 1, substantially filled in.
Yet another primary object is to realize insubstantial beam tilt variations which would otherwise occur with frequency in the elevation pattern of the beam. These beam variations or sway that occur are due to the fact that as the frequency varies above and below the design frequency, it becomes necessary to compensate for the so-called "phase taper" from layer to layer of the antenna that is produced. What occurs are beam tilt variations or sway across the channel that is involved.
It has been recognized by the present inventors that the problem of beam sway can be eliminated by center feeding the traveling wave antenna. This will be explained in detail hereinafter. Sufficient to say here that even though there is still a phase taper due to the frequency variation, that is, the variation from the design frequency to both frequencies below and above the design frequency, these phase tapers can be canceled. Thus, in accordance with the present invention by means of center feeding of the traveling wave antenna which involves, in effect, dividing the antennas into two halves, the phase taper in the top half is in the opposite direction from the phase taper in the bottom half. Consequently, the resultant phase taper is zero, eliminating any beam sway or variation with frequency.
In order to furnish some background material with respect to the invention, reference may be made to Masters patent U.S. Pat. No. 2,947,988 in which a disclosure is made of a traveling wave antenna particularly suitable for high frequency transmission. Whatever advantages reside in the provisions of that traveling wave antenna, it does not accomplish the objectives of the present invention, inasmuch as it does not produce the necessary high beam tilt along with the other mandated characteristics for the elevation pattern. For example, it does not realize the aforenoted objective achieving substantially no null variations in the elevation pattern for the beam. This is because the Masters scheme for realizing beam tilt functions by dependency on quarter-wave length spacing.
It should be noted that the term "illumination" in the antenna art is defined as a continuous relationship, shown herein by a typical known graph, which is a plot, as seen in FIG. 2, of relative radiated amplitude and phase between layers of the antenna. Since a traveling wave antenna, by definition, consists of matched layers that exhibit an attenuation producing radiation, and these layers are separated by some fraction of a wave length which creates a constant phase relationship from layer to layer, illumination here is in the form of a continuous straight line (constant slope).
Also, it should be understood that the elevation pattern of FIG. 1, for a UHF end fed traveling wave antenna produces the typical differential gain versus elevation angles across the band that would be expected. Thus, what results is a five DB variation in signals strength across the band at certain elevation angles. This is unacceptable for two reasons; first, it creates a high slope in the signal strength from one end of the band to the other at the receiver or which the automatic gain control must compensate; secondly, and most importantly, it spills energy outside of the Grade B contour which is imposed; that is, by the "coverage fence" according to the FCC rules.